Semiconductor material (wide-gap semiconductor material) such as silicon carbide (SiC) having a band gap wider than silicon recently attracts attention as semiconductor material suitable for semiconductor devices used under environments requiring a high breakdown voltage. For example, SiC has excellent characteristics of breakdown electric field strength, which is about ten times higher than silicon (Si), and can realize higher backward voltage rejection characteristics.
A pn junction diode, i.e., a bipolar type semiconductor device, fabricated by using SiC as a semiconductor material (hereinafter, “made of SiC”) can realize far better performance than a pn junction diode fabricated by using Si as a semiconductor material (hereinafter, “made of Si”).
For example, compared to a pn diode made of Si, if a pn junction diode made of SiC has a 10-kV or higher breakdown voltage, the pn junction diode has a forward voltage less than or equal to about ¼, operates at higher speed since the reverse recovery time corresponding to the speed at the time of turn-off is less than or equal to about 1/10, and can reduce electric power loss to about ⅙ or less of the pn junction diode made of Si. Therefore, the pn diode made of SiC is expected to significantly contribute to energy saving (see, e.g., Non-Patent Literature 1).
A switching device realizing a higher breakdown voltage by using SiC as a semiconductor material (hereinafter, “high-voltage semiconductor switching device”) can also significantly reduce electric power loss as compared to a switching device made of Si. Therefore, the high-voltage semiconductor switching device made of SiC is also expected to significantly contribute to energy saving.
FIG. 11 is a cross-sectional view of a conventional switching device. Bipolar transistors made of wide-gap semiconductor materials such as insulated gate bipolar transistor (SiC-IGBT) depicted in FIG. 11 and SiC-MOS accumulated channel gate bipolar transistor (MAGBT) are developed as switching devices made of SiC and the characteristics thereof are disclosed (see, e.g., Patent Document 1 and Non-Patent Literatures 2 to 4).
In the SiC-IGBT depicted in FIG. 11, reference numerals 1001 to 1010 and 1020 denote an n−-drift layer, a p-well layer, a p+-contact layer, an n+-emitter layer, a gate electrode, an emitter electrode, a gate insulating film, an n-buffer layer, a p+-collector layer, and a collector electrode, a Junction Field-Effect Transistor (JFET) area, respectively.
However, currently, Si-IGBT made of Si is often used as a switching device required to have a high breakdown voltage because of supply of large electric power and medium electric power and the Si-IGBT made of Si is frequently used in various application fields.
The Si-IGBT is realized and made into a product to the extent of 6-kV class breakdown voltages, for example. On the other hand, the Si-IGBT having a breakdown voltage greater than or equal to a 6-kV class leads to deterioration in another characteristic such as reduction in electric power loss and is difficult to satisfy both other characteristics and high breakdown voltage. Therefore, the Si-IGBT realizing a breakdown voltage greater than or equal to a 6-kV class is not yet made into a product.
For example, the SiC-IGBT as depicted in FIG. 11 realizes a high breakdown voltage of a 13-kV class, which is difficult to realize with the Si-IGBT, and realizes the usage under a high-temperature environment of 200 degrees C., which is difficult to realize with the Si-IGBT. The SiC-IGBT has lower on-resistance in an energized state as compared to a unipolar type switching device such as SiC-MOSFET having a 10 kV class breakdown voltage (see, e.g., Non-Patent Literature 2).
For example, while the on-resistance per unit area of the SiC-MOSFET having a 10-kV class breakdown voltage is about 100 mΩ-cm2, a SiC-IGBT having a 13-kV class breakdown voltage realizes a considerably lower on-resistance per unit area of 22 mΩ-cm2. SiC-IGBT operates at extremely high speed. For example, a turn-off time of the SiC-IGBT is about 150 ns and the off-operation of the SiC-IGBT is reduced to 1/10 or less as compared to the Si-IGBT having a 6-kV class breakdown voltage already made into a product.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-223220
Non-Patent Literature 1: Sugawara, Yoshitaka, “SiC power devices for large electric power conversion”, Oyo Buturi, The Japan Society of Applied Physics, 2001, Vol. 70, No. 5, pp. 530-535
Non-Patent Literature 2: Das, M. K., et al, “A 13 kv 4H—SiC n-channel IGBT with Low Rdiff, on and Fast Switching” (Switzerland), Material Science Forum, 2009, Vols. 600 to 603, pp. 1183-1186
Non-Patent Literature 3: Asano, K., et al, “A Novel Ultra High Voltage 4H—SiC Bipolar Device: MAGBT”, Proceedings of 19th International Symposium on Power Semiconductor Devices and ICs, 2004, pp. 305-308
Non-Patent Literature 4: Sugawara, Y., et al, “12.7 kV Ultra High Voltage SiC Commutated Gate Turn-off Thyristor: SICGT”, Proceedings of 19th International Symposium on Power Semiconductor Devices and ICs, 2004, pp. 365-368